Thursday, July 12, 2012

Harpoon motors and Estes retainer issues

The other day, I went to Hobby Lobby hoping to use a coupon on an F50-6 motor. It appears I have no will power even though SU motors are more expensive than reloads. They had no F50s (i.e. they sold the one they keep on the racks) so I bought an F26-6FJ, which is also a recommended motor for the Ventris. When I got home, I began thinking about the previous two flights. The F32 flight went well but there was some weather cocking. The F52 flight was arrow straight, but a -6 delay was a tad long. I had concerns about an F26 so I went back to RockSim. I now know the F26-6 is not optimal for my kit and have serious doubts about any Ventris. Admittedly, mine is about 2.5 oz heavier than the projected weight (18 vs.15.6 oz.).  I added a hefty swivel to the 'chute, used a quicklink, added rail buttons, etc.  The loaded weight of 21.5 oz is perilously close to the unsafe range based on the thrust-weight chart on ROL Info-Central.  With a 5' rail and a -4 delay, you could still probably use the F26 but I'll save my -6 for another rocket on another day.

I decided an F35 in the 24/60 case would be a better choice.  I even have an adapter that I built for the F32 flight. [insert sound of a vinyl record scratching to a halt]  Luckily, I test fit the motor - the retainer cap wouldn't screw on! Evidently, the wider thrust ring/closure was just large enough that it wouldn't fit.  Before I show my solution, I also became concerned about the AeroTech HP casings which have an even wider thrust ring (but obviously no adapter is required). I tested a 29/180 case and the retainer will screw on.  However, it doesn't have many threads holding it in place.  Now that I installed one of these retainers on my NCR Archer, I am contemplating using a washer retainer as insurance.

Well, on to my three-piece 'cut and paste' retainer.  A retainer for the 24/60 needs three things: 1) a tube to center the motor; 2) a thrust ring to keep the motor from flying through the rocket; and 3) a retainer to keep the case from flying out the back on ejection.  You can see these parts in the attached photo.  For #1, I cut a chunk off a spent 29mm SU motor. Item #2 needed to be thin but sturdy, so I made one from a metal lid from a small bottle.  The case might stay in the rocket as it is very slightly larger than the hole in the retainer but I am not 100% sure it wouldn't pop through.  So, I made a rear retainer from a plastic bottle cap. These parts will not be attached to one another and are all installed during motor installation. A properly sized washer would be nicer than the metal bottle cap but my local Home Depot won't have one.  The plastic rear retainer may end up being single-use, but there are always more pills to take. Problem solved.